Definition.—Stephanida with a simple dipleuric or bilateral ring, smooth or thorny, without branched spines and basal feet.
The genus Zygocircus and the following Dendrocircus differ from the two preceding older genera in the bilaterally symmetrical or dipleuric form of the sagittal ring. Whilst in Archicircus and Lithocircus the two sagittal halves or bows of the ring, the dorsal and ventral bow, are equal (therefore the fundamental form amphithect or diphragmatic), here both bows become distinctly different; the dorsal bow is constantly more straight (often vertical), the ventral bow more convex (obliquely ascending). This dipleuric differentiation is most important, as it is transmitted to the greater number of Nassellaria by heredity.
1. Zygocircus sagittalis, n. sp.
Gate ovate. Ring half ovate or nearly triangular, with three curved edges and three prominent corners, one apical and two basal protuberances. Dorsal rod vertical, twice as long as the horizontal basal rod. Ventral rod convexly curved.
Dimensions.—Diameter of the gate 0.05 to 0.07; height of the tubercles 0.01 to 0.015.
Habitat.—Central Pacific, Stations 265 to 268, depth 2900 fathoms.
2. Zygocircus trigonus, n. sp.
Gate triangular. Ring triangular, with three curved edges and three prominent corners, which are prolonged into nine divergent conical spines; three spines arising from the three edges of each corner. Dorsal rod vertical; ventral and basal rods curved and convergent.
Dimensions.—Diameter of the gate 0.11 to 0.13; length of the spines 0.01 to 0.015.
Habitat.—Indian Ocean, Sunda Archipelago (Rabbe), surface.